Anatomy Lesson: Inside Subwoofers

From cabinet to cone, here's how one Velodyne subwoofer rounds out your sound with thunderous bass.

YOU NEED the acoustical foundation of a subwoofer to replicate an earth-rumbling movie theater or a spine-tingling concert hall—or to take home gaming sessions to the next level of realism. High-quality bass extends the frequency range of your loudspeakers, providing a more realistic and pleasing listening experience. To help you understand what’s involved in creating the low end, I’ve dissected Velodyne’s Digital DrivePLUS subwoofer. No two subwoofer models are identical, but the parts described here are included in most. Now when you go to complete your sound system by purchasing a subwoofer, you’ll understand the salesperson’s lingo.

Responsible for producing the bass energy from a subwoofer. The surround supports the front edge of the cone while allowing movement. The dust dome keeps foreign particles out of the voice coil area.

2. SPIDER. Responsible for supporting the neck of the cone, as well as the front edge of the voice coil. Returns the cone and voice coil to the rest position after a signal from the power amplifier moves the cone.

3. FRAME. Holds the complete speaker driver structure. It’s comparable to the frame on a car, and the build quality can have drastic effects on the performance of a subwoofer.

4. VOICE COIL. A coil of wire wrapped around a cylindrical “former” and attached to the neck of the cone of the subwoofer driver that converts the current of a signal produced by the amplifier into a mechanical force that moves the cone.

5. MOTOR TOP PLATE. A piece of steel that helps focus the field from the magnet through the voice coil windings.

6. FRONT PANEL DISPLAY AND CONTROLS. Make setup and use of the subwoofer much easier and convenient. The subwoofer must have its own power amplifier to facilitate this feature.

7. MAGNET. A piece of magnetized material that generates the constant magnetic field that interacts with the varying current in the voice coil to move the cone.

8. MOTOR BACK PLATE. A piece of steel that completes the magnetic circuit from the back side of the magnet to the inside of the top plate, helping to focus the magnetic field through the voice coil.

9. CABINET. The outer structure that houses and protects all of the internal electronics and parts. Its main purpose is to stop sound radiated by the back of the subwoofer driver from interfering with the front radiated sound. The cabinet includes the baffle, a mounting board for the driver, as well as top, bottom, sides, and rear. Gaskets are used to seal openings and eliminate rattling where parts touch. Some cabinets may include auxiliary resonating devices such as ports or passive radiators as well.

10. BRACE. Higher-quality cabinets include solid bracing of some sort to rigidify the cabinet.
11. LINING MATERIALS. Materials such as Dacron and fiberglass are frequently used to ensure the cabinet acts purely as an air volume and has no reflections inside of it that can cause distortion to the emitted sound.

12. POWER AMPLIFIER. Most subwoofers come with a power amplifier, which will include the appropriate equalization, if any, for the model, as well as all the controls and connections necessary to properly mate the subwoofer to your sound system.

Important Specs

Woofer Size. All other factors being equal, a larger cone moves more air. More air movement means that your subwoofer can play louder.

Excursion. Not all woofers of the same size have the same excursion. Greater excursion allows the cone to move more air. Greater air motion equates to greater bass energy to be reproduced, whether it is in level or lower frequency.

Frequency Response. This describes how evenly a subwoofer can reproduce its range of sounds. Subwoofers are typically designed to reproduce frequencies from below 20 Hz to above 100 Hz, producing this band with more accuracy and authority than smaller satellite speakers.

Distortion. More or deeper bass doesn’t mean very much if it is produced with large quantities of distortion. Typically this will show itself as “slow” bass,” “muddy bass,” “overhang,” or even hearing a higher frequency than is intended to be reproduced.

Maximum Output. Because a powered subwoofer (one with an amplifier built in) has been designed to have a driver, cabinet and amplifier that work in concert, specifications such as sensitivity, nominal impedance, and power handling are not very important. But the end result, the maximum output capability, is very important. Most times, this needs to be experienced first-hand.